A converter of a power converter for Shinkansen is composed of a diode clamp type three-level circuit, in many cases.
On the other hand, recently a low loss device such as a silicon carbide device has been proposed. Accordingly, it is expected in future to miniaturize a converter, by applying a low loss device such as a silicon carbide device to the converter.
However, there is no device which can withstand a high voltage, in silicon carbide devices and so on provided at present. For this reason, the serialization of devices, or a multilevel configuration in which existing silicon devices and silicon carbide devices are combined are required. Out of the two measures, the serialization of devices has problems such as increase in resistance loss, increase in the number of devices, and necessity for balance control. For this reason, the multilevel configuration is practical under the present circumstances.
As a multilevel configuration, a flying capacitor system has been proposed, in which the number of switching devices for the number of output voltage levels is small. However, the relevant system causes the number of capacitors to be large.
As a multilevel configuration to suppress the number of capacitors, a diode clamp system has been proposed. In the diode clamp system, since a balance circuit for a filter capacitor voltage is required, there is a possibility that the volume thereof increases. In addition, there are a cascade system which AC input/output points of single-phase full bridge converters (inverters) are connected in series, and a gradation control system.
However, in the conventional technology, it is possible to withstand a high voltage, and the number of capacitors can be decreased, but the number of switching devices for the number of output voltage levels increases. Accordingly, it is difficult to miniaturize the apparatus.